1. Field of the Invention
This invention generally relates to a circuit and method for driving a device, and more particularly to a circuit and method for driving a display panel.
2. Description of the Related Art
FIG. 1 shows a schematic diagram of a conventional TFT LCD (thin-film-transistor liquid crystal display) device 10. The LCD device 10 includes an LCD panel 12, a control circuit 14, a first driving circuit 16, a gate driving circuit 18 and a power supply circuit 22. The LCD panel 12 is composed of two substrates and a liquid crystal layer interposed between the two substrates. A plurality of data lines 24, a plurality of gate lines 26 perpendicular to the data lines 24, and a plurality of thin film transistors 28 arranged as a transistor array are disposed on one of the two substrates. The transistors 28 arranged at each column in the transistor array have their sources electrically connected to each of the data lines 24, and the transistors 28 arranged at each row in the transistor array have their gates electrically connected to each of the gate lines 26. In addition, a capacitor 30 is formed between the drain of the transistor 28 and a common voltage VCOM. The power supply circuit 22 and the first driving circuit 16 are constructed as a source driving circuit.
After the control circuit 14 receives a horizontal synchronization signal Hsync and a vertical synchronization signal Vsync, it outputs corresponding control signals to the first driving circuit 16, the gate driving circuit 18 and the power supply circuit 22. The power supply circuit 22 is used for providing a plurality of level voltages V0 to Vn and for selectively transmitting the level voltages V0 to Vn to the first driving circuit 16 according to display data 32 and the control signals outputted from the control circuit 14. The first driving circuit 16 can receive the level voltages and respectively drive each data line 24 according to the received level voltages and the control signals outputted from the control circuit 14, whereby controlling the voltage difference between the two ends of each capacitor 30 and therefore changing the gray level of each pixel on the LCD panel 12. The gate driving circuit 18 can respectively output scanning pulses to the gate lines 26 according to the corresponding signals generated by the control circuit 14, whereby turning “on” or “off” the transistors 28.
U.S. Patent Publication No. 2003/0234757, published on Dec. 25, 2003, discloses a first driving circuit 16 as shown in FIG. 2. Now referring to FIGS. 1 and 2, FIG. 2 shows a detailed circuit of the first driving circuit 16 connected to the power supply circuit 22 and one row of transistors 28. The power supply circuit 22 comprises a plurality (only six shown in FIG. 2) of multiplexers MUX3 to MUX8. According to the control signals D3 to D8 outputted from the control circuit 14, each of the multiplexers MUX3 to MUX8 can select one of the level voltages V0 to Vn from a voltage bus 66 and then output the selected level voltage to the first driving circuit 16. The first driving circuit 16 comprises a plurality of operational amplifiers 44 and a plurality of switches 78 for controlling the current paths, wherein each switch 78 is respectively disposed between each operational amplifier 44 and each data line 24 (e.g. DL3 to DL8). When the gate line GL3 receives one scanning pulse from the gate driving circuit 18, each transistor 28 can be turned “on”; meanwhile, each operational amplifier 44 receives one of the level voltages V0 to Vn respectively from each multiplexer MUX3 to MUX8 and then drives each data line 24 to the voltage level of each received level voltage, whereby controlling the voltage difference between the two ends of each capacitor 30 and thus changing the gray level of each pixel on the LCD panel 12.
However, since the operational amplifiers 44 have different offsets affecting the actual output voltages, the voltage levels outputted from the operational amplifiers 44 are different even if the operational amplifiers 44 receive the same level voltage from the multiplexers MUX3 to MUX8; therefore, the voltage differences between the two ends of the capacitors 30 are different, which may cause uneven display under the same gray level and thus deteriorate the display quality. Accordingly, the switches 78 are utilized to solve the problem of uneven display.
FIG. 3 shows the voltage waveforms at the output terminal VM of the multiplexer MUX3 and the data line DL3 shown in FIG. 2 for illustrating the operation of the first driving circuit 16. It is assumed that the initial voltages of the output terminals VM of the multiplexers MUX3 to MUX 8 and the data lines DL3 to DL8 are Vn, and the target voltages of the same are V0; further, the scanning line GL3 receives one scanning pulse to turn “on” the transistors 28 arranged at the same row.
During the time t0 to t1, the switch 78 is switched to electrically connect the terminals E1 and E2 such that the operational amplifier 44 can drive the data line DL3 from the voltage Vn toward V0 according to the voltage change at the output terminal VM of the multiplexer MUX3.
During the time t1 to t2, the switch 78 is switched to electrically connect the terminals E1 and E3 such that the data line DL3 can receive the level voltage V0 directly from the output terminal VM of the multiplexer MUX3. In this period, all the data lines DL3 to DL8 receive and are directly driven by the level voltages V0, which are respectively selected from the voltage bus 66 through the multiplexers MUX3 to MUX 8, such that the uneven display caused by different offsets of the operational amplifiers can be eliminated; further, the data lines DL3 to DL8, therefore, can be precisely driven to the level voltage.
However, the operational amplifier generally has a good driving ability and is able to pull the voltage level of the data line DL3 rapidly and closely toward the voltage level of the level voltage V0 prior to time t1. Therefore, the period, i.e. time t0 to t1, is too long for the operational amplifier to drive the data line DL3, which may cause additional power consumption of the operational amplifier.
Now referring to FIGS. 1 and 4, FIG. 4 shows a schematic diagram of the gate driving circuit 18 connected to one column of transistors 28. The gate driving circuit 18 comprises a shift registering circuit 80, a level shifting circuit 82 and a buffering circuit 84. The shift registering circuit 80 is composed of a plurality of shift registers 81 series-connected to each other, and used for receiving a gate starting pulse Y and a gate shifting clock CLKY from the control circuit 14 and then sequentially outputting the gate starting pulse Y to the level shifting circuit 82 according to the gate shifting clock CLKY. Each of the shift registers 81 can be implemented by D-type latch. The level shifting circuit 82 comprises a plurality of level shifters 83 for sequentially receiving the gate starting pulse Y and converting the received gate starting pulse Y into a scanning pulse, which is appropriate to drive the gate of the corresponding transistor 28. The buffering circuit 84 comprises a plurality of buffers 85 for sequentially receiving the scanning pulse and outputting the received scanning pulse to the gate of the corresponding transistor 28 through the gate lines GL0 to GLn whereby sequentially turning “on” the transistors 28.
However, in the gate driving circuit 18, the scanning pulses outputted by the buffers 85 are not identical and have different driving capacities. Therefore, when the gates of the transistors 28 receive the scanning pulses having different driving capacities, especially having weaker driving capacities, the capacitors 30 may be charged to different voltage levels and thus cause uneven display under the same gray level.
Accordingly, the present invention provides a circuit and method for driving a display panel so as to solve the above-mentioned problems in the art.